1. Field of the Invention
The present invention relates to an optical network system the adopts a passive optical network (PON) in which data of different bit rates coexist, and a method of data transmission in the passive optical network.
2. Description of the Related Art
Currently as a subscriber loop optical fiber network system in which premises such as general households are the object, there is known for example a system in which an optical line terminal (station) provided on an aggregated station arranged on a telephone station or the like, and an optical network unit arranged on a plurality of subscriber households, are connected by a transmission line using an optical fiber. Amongst these, a configuration that branches an optical signal transmitted from an aggregated station to an optical transmission line into a plurality of optical signals by a power splitter being a passive element, and connects the optical network unit of each of the premises to the branch target, is called a passive optical network (PON) system.
This PON system is utilized as a system capable of receiving data at high speed between the optical line terminal and the plurality of optical network units. Furthermore, as one form of a transmission network which uses the PON system, there is the system configuration as shown for example in FIG. 6. This comprises an optical line terminal (OLT) 110, N (where N is an integer of two or more) optical network units (ONU) 120-1 to 120-N corresponding to the respective subscribers, optical transmission lines 101 and 101-1 to 101-N connecting between each of these, and a power splitter 102.
The OLT 110 is of a configuration furnished with a necessary transmission control function which, in order to perform information distribution and the like, converts an electrical signal into an optical signal and transmits this to the respective ONUs 120-1 to 120-N in a predetermined frame format, or converts data that has been sent as an optical signal of a predetermined frame format from the respective ONUs 120-1 to 120-N into an electrical signal. Furthermore, the respective ONUs 120-1 to 120-N are configured to perform communication control such as communication with the OLT 110, and conversion between an optical signal and an electrical signal.
Here, between the OLT 110 and the power splitter 102, the upstream and downstream data transmission uses a single optical transmission line 101 and performs communication in both directions by wavelength division multiplexing (WDM). The direction from the OLT 110 to the respective ONUs 120-1 to 120-N is the downstream direction, and the direction from the respective ONUs 120-1 to 120-N to the OLT 110 is the upstream direction. For example, the downstream frame from the OLT 110 to the respective ONUs 120-1 to 120-N is transmitted at a single bit rate as an optical signal of 1.49 □m by time division multiplexing (TDM). In the respective ONUs 120-1 to 120-N, the frame synchronization information and the administrative information in the downstream frame are detected, and based on this, the data for the time slot which is individually assigned beforehand is taken out. On the other hand, in the upstream frame from the respective ONUs 120-1 to 120-N to the OLT 110, this is transmitted from the respective ONUs 120-1 to 120-N at a timing which is given by the OLT 110. That is to say, in the upstream frame 110 from the respective ONUs 120-1 to 120-N, this is transmitted as an optical signal of 1.31 □m band by a time division multiple access (TDMA) method at a timing at which these do not collide.
The power splitter 102 distributes (power splits) the downstream frame from the single optical transmission line 101 to a plurality of optical transmission lines 110-1 to 110-N. Furthermore, it serves the role of aggregating (combining) the upstream frame from the plurality of optical transmission lines 101-1 to 101-N to a single optical transmission line 101.
Incidentally, regarding the above such PON system, in general, use with the purpose of providing a low speed service of a telephone level has been examined. However recently, rather than a telephone service, it is necessary to provide a high speed communication service of a high speed video telephone service or video conferencing service and the like, and there is a demand to provide data of a low speed bit rate (for example a low speed service of a telephone or the like), and data of a high speed bit rate (for example a high speed service of a video telephone service or a vide conferencing service, or the like), on a single PON system.
As a conventional technique for realizing a PON system in which such data of different bit rates coexist, for example in Japanese Unexamined Patent Publication No. 2005-33537 there is disclosed a method in which the upstream direction communication frame comprises a plurality of burst slots having the same duration, and the respective ONUs send a number of data cells corresponding to each of the burst rates, with respect to the burst slots specified by a control cell from the OLT, and the upstream direction communication frame transports data cells at different bit rates depending on the burst slot position.
In the conventional PON system as described above, for example in the case where an ONU that corresponds to data of a high speed bit rate is newly added to an existing PON system that corresponds to data of a low speed bit rate, it is necessary for the ONU that corresponds to the high speed bit rate to satisfy the level diagram of the existing system, and also this must not have an influence on the data transmission of the existing system. For example, in a G-PON_Class_B+ specified by ITU-T_G.984.2, since the minimum reception rate is −28 dBm, it is necessary for the ONU that corresponds to the newly added high speed bit rate to also maintain a minimum reception rate of −28 dBm.
However, for the ONU that corresponds to the high speed bit rate, since the band width is wide, this is susceptible to influence of noise, and compared to the ONU that corresponds to the low speed bit rate, receiver sensitivity is poor. For example, as shown in FIG. 7, the minimum reception level for the bit rate of the ONU that corresponds to 10 gigabits per second (Gbps) is only around −25 dBm at 10 Gbps. Therefore, if an ONU that corresponds to 10 Gbps is added to an existing system that corresponds to a bit rate of 2.5 Gbps or the like, then in the ONU that corresponds to 10 Gbps it becomes difficult to ensure a minimum reception rate of −28 dBm specified by the aforementioned G-PON_Class_B+, and a problem arises in that the level diagram of the existing system cannot be satisfied. In order to satisfy the level diagram of the existing system, it is necessary to improve the receiver sensitivity of the ONU that corresponds to 10 Gbps to 3 dB or more, without having an influence on the reception processing of the data in the ONU that corresponds to 2.5 Gbps.